The present invention relates to a method for the preparation of a planographic printing plate by the silver complex diffusion transfer method.
The principles of the silver complex diffusion transfer reversal process, hereinafter called DTR-process, have been described e.g. in U.S. Pat. No. 2,352,014 and in the book "Photographic Silver Halide Diffusion Processes" by Andre Rott and Edith Weyde--The Focal Press--London and New York, (1972).
In the DTR-process non-developed silver halide of an information-wise exposed photographic silver halide emulsion layer material is transformed with a so-called silver solvent into soluble silver complex compounds which are allowed to diffuse into an image-receiving element and are reduced therein with a developing agent, generally in the presence of physical development nuclei, to form a silver image having reversed image density values ("DTR-image") with respect to the black silver image obtained in the exposed areas of the photographic material.
A DTR-image bearing material can be used as a planographic printing plate wherein the DTR-silver image areas form the water-repellant ink-receptive areas on a water-receptive ink-repellant background. For example, typical lithographic printing plates are disclosed e.g. in Japanese Examined Patent Publication (Kokoku) 30562/73, Japanese Unexamined Patent Publications (Kokai) Nos. 21602/78, 103104/79, 9750/81etc.
The DTR-image can be formed in the image-receiving layer of a sheet or web material which is a separate element with respect to the photographic silver halide emulsion material (a so-called two-sheet DTR element) or in the image-receiving layer cf a sc-called single-support-element, also called mono-sheet element, which contains at least one photographic silver halide emulsion layer integral with an image-receiving layer in waterpermeable relationship therewith. It is the latter mono-sheet version which is preferred for the preparation of offset printing plates by the DTR method. Thus, it is known e.g. from the United Kingdom Patent Specification 1,241,661 to produce a planographic printing plate consisting of a sheet material comprising an outer hydrophilic colloid layer on the surface of which there is concentrated a silver image stemming from an underlying exposed silver halide emulsion layer by the silver complex diffusion transfer reversal process.
The developing agent or a mixture of developing agents can be present in an alkaline processing solution and/or in the photographic silver halide emulsion layer material itself. In case the developing agent or a mixture of developing agents is contained in the photographic silver halide emulsion material, the processing solution can be merely an aqueous alkaline solution, substantially free of developing agents, that initiates and activates the development, such alkaline solution being referred to herinafter as "activator solution". On the other hand the express ions "developing solution" and "development type DTR element" as used herinafter are meant as being restricted to "solutions containing developing agents" and "DTR element to be developed with a solution containing developing agents".
Preferred silver halide solvents are watersoluble thiosulphate compounds such as ammonium and sodium thiosulphate, or ammonium and alkali metal thiocyanates. Other useful silver halide solvents (or "complexing agents") are described in the book "The Theory of the Photographic Process" edited by T. H. James, 4th edition, p. 474-475 (1977), in particular sulphites and uracil. Further interesting silver halide complexing agents are cyclic imides, preferably combined with alkanolamines, as described in U.S. Pat. No. 4,297,430 and U.S. Pat. No. 4,355,090. 2-mercaplobenzoic acid derivatives are described as silver halide solvents in U.S. Pat. No. 4,297,429, preferably combined with alkanolamines or with cyclic imides and alkanolamines.
Usually when a photographic DTR-material is intended to serve as lithographic printing plate the developing or activator solution is followed by a so-called neutralization solution containing buffer substances to neutralize the alkaline DTR-material after treatment with the developer or activator bath.
According to a well known conventional embodiment the silver halide emulsion contained in the DTR element is orthochromatically sensitized with a conventional cyanine or merocyanine dye so that said DTR element can be exposed on a process camera using an ordinary light source, e.g. tungsten light. In this conventional embodiment no serious problems of silver halide emulsion sensitivity arise. Commercially available DTR materials according to this conventional method which can be processed to lithographic printing plates are marketed e.g. by MITSUBISHI PAPER MILLS LTD under the trade name SILVERMASTER and by AGFA-GEVAERT N.V. under the trade name SUPERMASTER.
According to more recent developments new types of DTR elements are used as recording materials for phototype-setting and image-setting devices which employ laser beams as their output energy source. According to these so-called "direct to plate" methods these laser-exposed DTR materials are transformed in direct-to-use printing plates containing type- and image information, thus avoiding the intermediate steps as in conventional pre-press work flow.
As a consequence these DTR elements must show a spectral sensitivity matching as closely as possible the emission wavelength of the laser beam in question.
For example, lithographic DTR elements suited for exposure by helium-neon laser (HeNe) or light emitting diode (LED), both devices emitting in the red region of the visual spectrum, are disclosed in U.S. Pat. No. 4,501,811 and Japanese Unexamined Patent Publication (Kokai) Nos. 71055/84 and 75838/85. A commercially available DTR material spectrally sensitized for HeNe exposure is marketed by AGFA-GEVAERT N.V. under the trade name SETPRINT HeNe-plate PP 410.
On the other hand semiconductor lasers, also called laserdiodes, show some advantages compared to other laser types such as low cost price small size and long life time. Generally the emission wavelength of these semiconductor laser beams is longer than 700 nm and mostly longer than 750 nm. A commercial image-setter using infra-red laserdiode source was announced by XENOTRON Co in "Ifra Newspaper Techniques" No 7 (oct. 1985) p. b 27. Photographic materials appropriate for exposure by devices employing such laserdiodes must be sensitized for the near infra-red region of the radiation spectrum. Suitable photographic materials lo be used with semiconductor laser device are disclosed in Japanese Unexamined Patent Publication (Kokai) No 61752/85 and U.S. Pat. No. 4,784,933. Commercial infra-red sensitized phototypesetting film and paper were announced by EASTMAN KODAK Co in "Proceedings Lasers in Graphics, Electronic Publishing in the 80's", Vol. 2 (sept. 1985) p. 508.
A photographic DTR material normally comprises a silver halide emulsion mainly consisting of silver chloride in order to obtain a sufficiently high rate of solution of the silver halide and a satisfactory gradation necessary for graphic purposes. Only a small amount of silver bromide and/or silver iodide usually not exceeding 5 mole % is present. However as a consequence of the lower energy output of some lasers, e g. laserdiodes, the sensitivity of these conventional silver chloride DTR emulsions is no longer sufficient when using those types of laser as exposure unit. Apart from the intrinsic lower sensitivity of silver chloride compared to silver bromide it is thought that some unfavorable factors, unimportant when sufficient exposure energy is present, begin to play a role at low energy output of the exposure unit. First, it is supposed that the DTR silver transfer speed is reduced somehow by interference with the spectral sensitizers. Secondly, when strong complexing agents, preferred for producing compact DTR silver, such as thiocyanate or a mixture of agents as described in U.S. Pat. No. 4,297,429 are used, part of the exposed silver chloride is not chemically developed in due time; this results in partial silver complexion and the production of dichroitic fog in the non-image areas. Both phenomena are even more pronounced in the case of the activator type of DTR material and processing solution composition than in the case of the development type.
When one tries however in the DTR material to substitute the silver chloride emulsion by a conventionally prepared emulsion containing chloride and bromide in the hope of gaining sensitivity nc sufficient transfer of DTR silver is achieved anymore. This is both the case in the development type of DTR element and in the activator type of DTR element As a result the maximal density and gradation of the DTR silver image are reduced resulting in a severe deterioration of the image quality and the printing endurance characteristics. After a limited length of printing run the DTR silver image is worn off and the ink acceptance and transfer becomes insufficient so that only a small amount of printed copies can be produced.
It is a purpose of the present invention to overcome the sensitometric difficulties encountered with DTR material and processing solution of both the developing and the activator type, especially in the case of low energy output of the laser exposure unit.